Figure 6. Na⁺ currents are diminished by repetitive depolarization.

A, the cell membrane was depolarized 100 times for 5 ms with a frequency of 20 Hz (first 30 current traces shown). The membrane holding potential (-57 mV) was adjusted to the recovery potential 20 s before the start of the stimulation protocol. No leak subtraction was applied. B, current amplitudes of traces elicited by membrane depolarizations for 5 ms with frequencies of 5 and 50 Hz. The holding potential was -57 mV. C, the proportional reduction of the current amplitude increased with depolarization frequency. For the various stimulation frequencies, the reduction was significantly larger at a holding potential of -57 mV than at -77 mV, as indicated by the asterisks. Numbers in parentheses represent the number of cells.